If a player gets injured and you have had only 1 substitution and there are 5 fit players on the bench,
what are the reasons because of which he may not get substituted ?

What is the algorithm for substitution?

For all my matches (those I watched), there is always a substitution when my players get injured if I still have my 3 or 2 or 1 more substitution available to be made.

The rules of substitution from what I have seem is position for position, in general.

What I meant general is defender for defender even if your center back is injured the next in line is a good side back, he will still get substituted in. However these position - wing back, defensive midfielder, attacking midfielder, winger - can be substituted by two different positions. (not 100% sure for wingers).

If a player is injured and you have only made 1 substitution, then he should be substituted & the bot will pick the best available to replace him with. If he wasn't replaced then it is a bug & you should report it in the bug reports forum.

Thanks then thats what I shall do.